This invention relates to a method for dry etching and, more particularly, to a method for etching a polycide film used for, for example, a gate electrode, with high anisotropy and high substrate selectivity without using flon type gases.
As the gate wiring material for LSIs, polysilicon has been used extensively. However, with an increasing demand for a higher device operation speed, silicides of refractory metals have come into popular use, with which a resistance value about one digit lower than that with the polysilicon may be achieved. When forming a gate wiring layer by using silicides of refractory metals, the recent tendency is to use a so-called polycide film, that is, to deposit a layer of silicides of refractory metal on a gate insulating film by the interposition of doped polysilicon layer (DOPOS layer) instead of directly depositing the silicide layer on the gate insulating film. In this manner, the resistance of the gate wiring material may be lowered by the deposition of the refractory metal silicide layer on the polysilicon layer, a proven wiring material, which is applied to an interface with the gate insulating film which tends to affect the device characteristics and reliability most seriously.
However, with such polycide film, difficulties have been newly raised in connection with dry etching, since it is required of the polycide film to present anisotropy with respect to both of two different types of materials. That is, due to the difference in vapor pressures of the halogen compounds yielded during the etching of the polycide film, the lower DOPOS layer is etched more quickly than the upper refractory metal silicide layer, or a reactive layer is produced at the interface between the DOPOS layer and the refractory metal silicide layer, with the result that undercuts or distortions may be produced in the pattern.
For example, in the case of a substrate in which, as shown in FIG. 6(A), a polycide film 15 consisting of a DOPOS layer 13 and a refractory metal silicide layer 14 is deposited on a silicon substrate 11 with the interposition of an insulating film 12 of, for example, silicon oxide, when the polycide film 15 is to be etched with a resist pattern 16 as a mask, shape abnormalities such as are shown for example in FIG. 7 may be experienced frequently. That is, an undercut 17 is produced below the refractory metal silicide pattern 14a formed anisotropically below the resist pattern 16, such that the produced DOPOS pattern 13d is of a width which is narrower than the desired pattern width.
These shape abnormalities cannot be tolerated especially with submicron devices since an offset region devoid of impurities may be formed during ion implantation for forming the source-drain region, or dimensional accuracy in the course of side wall formation for realizing an LDD structure may be lowered.
For realizing anisotropic etching, it may be contemplated to i) raise an ionic energy, ii) add a deposition gas to an etching gas system to produce a mixed gas to be used for side wall protection or iii) to produce a reaction product of an etched material and an etching gas and to use the reaction product for side wall protection. However, these measure are not without their demerits. That is, increase in the ion energy gives rise to recession of the resist or to damages, whereas addition of the deposition gas to the etching gas system results in pollution by particles. On the other hand, with side wall protection by the reaction product, uniform patterns cannot be produced since the effect of side wall protection is changed with the size of the etched area. Practically useful anisotropic etching cannot be realized unless these problems are overcome simultaneously.
An etching gas consisting mainly of a Flon base gas, such as Flon 113 (C.sub.2 Cl.sub.3 F.sub.3), has been used most extensively as the etching gas for a polycide film, as reported for example in "Semiconductor World", edited by Press Journal, October issue, 1989, pages 126 to 130. Since this gas contains both fluorine and chlorine atoms in its molecules, the etching reaction may proceed effectively in both the radical and ion modes, at the same time that highly anisotropic etching becomes possible as side wall protection is assured simultaneously by deposition of the carbon base polymer.
However, the Flon gases have been pointed out as causing destruction of an ozone layer surrounding the earth, and hence the manufacture as well as the use of the gas is to be prohibited in the near future. Thus it is incumbent in the field of dry etching to find a suitable substitute material for the Flon gases and to establish the technology of effective utilization of the substitute material.
It is also incumbent in the field of polycide film etching to establish high selectivity with respect to the etching base.
For example, for employing the polycide film for formation of a gate wiring, the etching base is frequently a silicon oxide layer for providing a gate insulating film. This silicon oxide layer should operate as a stop for etching. Recently, the film thickness of the gate insulating film has been reduced to an increasing degree and, since an overetching is usually practiced during the etching process for removing etching residues for the reason which will be explained subsequently, a process exhibiting superior etching base selectivity is indispensable.
The reason why the overetching needs to be practiced is hereinafter explained with reference to FIGS. 6(A) and 6(B). For example, with on etching the substrate shown for example in FIG. 6(A), etching residues 13b, 13c may be produced at the time point when the majority of the polycide film 15 has been etched and the refractory metal silicide pattern 14a and the DOPOS pattern 13a have been formed. These etching residues are produced to a more or less extent due to fluctuations in the etching conditions or in-plane nonuniformities of the etched material. Although not shown, when the polycide film is formed on a substrate having a step or steps, etching residues tend to be formed on the step or steps. These etching residues need to be removed by overetching. However, high selectivity is required because of the extremely thin thickness of the insulating film 12 as the etching base.
Recently, HBr has attracted attention as an etching gas in view of the demand for reducing the size of the device, finding a suitable substitute material for Flon gas and improving the etching base selectivity. For example, in "Digest of Papers, 1989, Second Microprocess Conference", page 190, a report is made of an example in which a satisfactory shape anisotropy has been realized by reactive ion etching of the n.sup.+ type DOPOS layer with the use of HBr. It is difficult with Br to produce a spontaneous etching reaction since Br has a larger atomic radius and is unable to invade easily into the crystal lattices or into the grain boundary of the etched material. However, it is possible with Br to produce an etching reaction when ion bombardment is accompanied so that Br may provide etchants useful for achieving the desired anisotropy. For this reason, various attempts have been made to use Br etchants and to set the bias power to an optimum value to procure a high selectivity with respect to the gate oxide film as the etching base for achieving satisfactory anisotropic etching.
It has however become apparent that, although successful results have been obtained to some extent with dry etching by HBr in connection with anisotropic etching of the DOPOS layer and etching base selectivity, the following disadvantages have been experienced on application of the dry etching by HBr to etching of the silicide film. That is, the etching chamber may be contaminated by refractory metal bromides sputtered out during etching of the refractory metal silicide layer and, since Br base radicals used as the etchant are intrinsically low in reactivity, the etching rate is drastically lowered as compared to that of etching with the use of the conventional Flon gases.